Presently, laser engravers employ two methods to move a laser beam relative to a workpiece in order to establish a pattern thereon. The first method is known as the raster scan method. In the raster scan method a laser beam having a defined width is horizontally swept across the workpiece. As the laser beam is horizontally swept across the workpiece it is also modulated to establish the pattern or a portion thereof on the workpiece. Following the completion of a horizontal sweep the laser beam is moved vertically relative to the workpiece by an incremental distance, typically the width of the laser beam, thereby positioning the laser beam for another horizontal sweep. The process of moving the laser beam horizontally and then vertically relative to the workpiece is repeated until the entire pattern is established on the workpiece. Exemplary of an apparatus that implements the raster scan method is U.S. Pat. No. 4,354,196, issued on Oct. 12, 1982, to Neumann et al., for a "Laser Engraving System with Massive Base Table and Synchronization Controls". In Neumann et al, a mirror is rotated at a constant angular velocity to horizontally sweep the laser beam. Vertical movement of the laser beam relative to the workpiece is accomplished by moving the carriage on which the workpiece is located.
The primary drawback associated with laser engravers that implement the raster scan method is that the time required to process a workpiece is a function of the area encompassed by the pattern and not the area of the pattern to be established on the workpiece. For example, suppose that the pattern to be engraved on a workpiece is an outline of a square having sides that are one foot in length and one inch wide. A raster scan laser engraver would have to move the laser beam over the entire one square foot of surface area encompassed by the outline of the square in order to establish the outline of the square on a workpiece. In contrast, the actual area engraved is approximately a third of a square foot. Consequently, laser engravers that implement the raster scan method are relatively inefficient in applications where the area encompassed by the pattern is relatively large in comparison to the area of the pattern itself.
The second method presently used to move a laser beam relative to a workpiece in order to establish a pattern thereon is characterized by the use of a high-performance galvanometer driven mirror assembly in combination with a diverging lens. The galvanometer driven mirror assembly typically includes a mirror suspended from a gimbal which permits the mirror to be rotated in any direction. One or more solenoids are used to rotate the mirror with respect to the gimbal. High performance is achieved in a galvanometer driven mirror assembly by restricting the degree to which the solenoids can rotate the mirror. However, restricting the degree to which the solenoids can rotate the mirror also restricts the area over which the beam can be moved for a given separation between the mirror and the workpiece. In order to expand the area over which the beam can be moved the aforementioned diverging lens is interposed between the mirror and the workpiece to expand or amplify the area over which the beam can be applied. A pattern is established on a workpiece by directing a laser beam to the mirror and rotating the mirror by an amount which takes into account the amplification caused by the diverging lens.
There are several drawbacks associated with laser engravers that employ a high-performance galvanometer driven mirror in conjunction with a diverging lens. Namely, such laser engravers are very sensitive to angular errors in the positioning of the mirror. In other words, a small angular error in the position of the mirror produces a relatively large error in the positioning of the laser beam on the workpiece due to, among other things, the amplification of the diverging lens. Concomitantly, the amplification of the diverging lens is dependent upon the distance between the mirror and the workpiece. Hence, such laser engravers are also sensitive to the distance separating the mirror and the workpiece. Further, the amplification provided by the diverging lens is directly related to its diameter which is, in turn, directly related to its expense. Consequently, applications that require a relatively large diverging lens in order to establish a pattern on a workpiece are prohibitively expensive.
Alternatively, a high performance, motor driven, single or multi-lens focus assembly can be combined with two galvanometer driven mirrors. In operation the galvanometer driver mirrors are rotated to position the laser beam on the workpiece. Such laser engravers are also expensive, and sensitive to relatively small errors in the angular position of the mirror and the distance between the mirror and the workpiece.
Another consideration with respect to laser engravers is the ability to control the depth of the cut made in the workpiece by the laser beam. The depth of the cut is directly related to the power of the laser beam and the speed with which the laser beam is being moved relative to the workpiece. For example, if the power of the laser beam is constant and the laser beam is being moved at a constant speed relative to the workpiece then a cut having a constant depth will be produced. The depth of the cut can be varied by varying the power of the laser beam and/or the speed with which the laser beam is being moved relative to the workpiece.
One method of producing a cut with a constant depth in laser engravers employing the raster scan method is to limit variations in the speed at which the laser beam is moved to points exterior to the workpiece and to maintain both the power of the laser beam and its rate of travel relative to the workpiece at constant levels while it is passing over the workpiece. More specifically, the laser beam is positioned a sufficient distance from the edge of the workpiece to allow acceleration of the laser beam to a constant velocity while it is still exterior to the workpiece. Consequently, once the laser beam engages the workpiece it is moving at a substantially constant velocity. By maintaining both the power of the laser beam and the speed of laser beam at constant levels a cut having a constant depth can be produced. Once the laser beam leaves the workpiece, its speed is reduced so that a return sweep can be made in the opposite direction. A drawback associated with maintaining a constant depth of cut in this fashion is the time, and hence the inefficiency, associated with accelerating and decelerating the laser beam when it is exterior to the workpiece. Another drawback associated with the aforementioned method is that it does not allow the depth of cut or cut profile to be varied. For instance, the aforementioned method cannot produce cuts having different constant depths. Further, the aforementioned method cannot produce a cut that is, for instance, initially shallow and becomes progressively deeper.
Another method for maintaining a constant depth of cut in laser engravers employing the raster scanning method is suggested by the apparatus for producing a uniform exposure of a medium on which an artwork master for a circuit board is to be established in the patent to Neumann, et al. In Neumann, the beam is swept across the workpiece by rotating a mirror at a constant angular velocity. As the beam is swept across the workpiece its horizontal velocity varies thereby producing a nonuniform exposure. The variation in the horizontal velocity of the beam is attributable to, among other things, the angular velocity of the mirror, the shape of the workpiece (typically planar) and the distance separating the mirror and the workpiece. In order to produce a uniform exposure the variation in the horizontal velocity of the beam is compensated for by modulating the power or intensity of the laser beam with a signal that is proportional to the horizontal velocity of the beam. Such a signal is produced by a phase comparator that is, in turn, part of a phase-locked loop that is used to compensate for positional errors that are attributable to, among other things, the aforementioned variation in the velocity of the beam. Based on the foregoing such an apparatus for producing a uniform exposure is sensitive to, among other things, the distance separating the mirror and the workpiece. Further, such an apparatus is not capable of varying the exposure. Additionally, such an apparatus is, typically, complicated, difficult to implement, and expensive.
In laser engravers that employ a galvanometer driven mirror in combination with either a diverging lens or a motor-driven multi-lens focus assembly maintaining a constant depth of cut is generally not a problem due to the responsiveness of such systems. More specifically, the relatively high acceleration and deceleration of the galvanometer driven mirror results in the laser beam being brought up to speed relatively quickly. Consequently, a constant depth of cut can be achieved in such an engraver by simply maintaining the power of the laser beam at a constant level during the cutting of a pattern in a workpiece. Such apparatuses are not, however, capable of varying the depth of cut. Further, the aforementioned drawbacks associated with laser engravers that employ a galvanometer driven mirror in conjunction with a diverging lens or motor-driven multi-lens focus assembly on the whole render such engravers undesirable for certain applications.
Based on the foregoing there exists a need for a laser engraver that can move the laser beam relative to the workpiece and control the depth of cut in a timely and efficient manner. Moreover, there exists a need for a laser engraver that is relatively insensitive to errors in the positioning of the laser beam or the workpiece, inexpensive to implement and reliable.